There is a known technique of protecting an occupant by an airbag apparatus mounted to a motorcycle. For example, Japanese Unexamined Patent Application Publication No. 2001-219885 discloses a technique of inflating an airbag to deploy it into a spacing between a vehicle component such as a head pipe and an occupant seat of a motor scooter. This technique presents the possibility of mounting an airbag apparatus to a motorcycle. However, there is a high requirement for complete protection of an occupant by pursuit of a rational inflation pattern of the airbag in the event of an accident.